Screw compressors have been conventionally widely used for applications of compressing refrigerant or air. For example, Japanese Patent No. 4140488 shows a screw compressor including a screw rotor on which a plurality of helical grooves are provided, and gate rotors each in which a plurality of gates are provided.
In the above screw compressor, the gate rotor rotates with the rotation of the screw rotor. The gates of the gate rotor relatively move from the start ends (ends at a suction side) toward the terminal ends (ends at a discharge side) of the meshed helical grooves, and the volume of a compression chamber which is closed gradually decreases. As a result, a fluid in the compression chamber is compressed.
In the compression chamber in the course of compression, an intermediate pressure refrigerant is ejected from the economizer port. This ejection can reduce the temperature of a discharge refrigerant in the screw compressor to a predetermined temperature or less to improve the performance of the screw compressor.